Photographic printing apparatus



Kpril 13, 1963 HIDEAKI AKIMA 3,177,764

PHOTOGRAPHIC PRINTING APPARATUS Filed Nov. 20, 1961 5 Sheets-Sheet 1 April 15, 1965 Filed Nov. 20, 1961 5 Sheets-Sheet 2 I FIG.3.

fidelity Spafiul frequenby FIG.4.

April 13, 1965 Filed Ndv. 20, 1961" HlDEAKl AKlMA PHOTOGRAPHIC PRINTING APPARATUS FIG. 5.

5 Sheets-Sheet 3 IN VENTOR.

April 13, 1965 HIDEAKI AKIMA PHOTOGRAPHIC PRINTING APPARATUS 5 Sheets-Sheet 4 Filed Nov. 20, 1961 FIG.6.

April 13, 1 HlDEAKl AKlMA 3,177,764

PHOTOGRAPHIC PRINTING APPARATUS Filed Nov. 20, 1961 5 Sheets-Sheet 5 A INVENTOR.

United States Patent 3,177,764 PHOTOGRAPHIC PRINTING APPARATUS Hideaki Akima, Tokyo, Japan, assignor to Canon Camera Co., Ltd, Tokyo, Japan Filed Nov. 20, 1961, Ser. No. 153,576 Claims priority, application Japan, Nov. 21, 1960,

2 Claims. (Cl. 88-24) The present invention relates to improvements in a photographic printing apparatus and more particularly to an improved photographic printing apparatus wherein the electronic techniques are effectively utilized, so that the contrast of a printed image can be automatically improved, and a desirable exposure value as well as viewing of the finished positive image on a fluorescent surface of a monitor cathode ray tube can be effectively attained.

In the conventional photographic printing method, improvement of sharpness of a photographic print has been attained merely by printing the image on a high gamma photographic paper and increasing the contrast. However, since the straight range of the characteristic curve of density of a high gamma photographic paper is narrower than that of a low gamma photographic paper, in every case where the original negative which has been photographed at a position where the difference between brightness and darkness is remarkable or the original negative has been photographed at a normal brightness under controlled conditions, the difference between maximum and minimum intensity of printing light exceeds, when the printing is carried out, the straight range of the said characteristic curve of density of the high gamma photographic paper, whereby the image corresponding to detail at a position of high brightness becomes white, and the image corresponding to shadows becomes black, thus making true reproduction of the detail impossible. For obtaining sharpness of a photographic print, gross contrast and detail contrast must be reproduced with uniform fidelity. However, in general, this improved condition cannot be satisfied, because the deficiency in sharpness due to lens and photographic sensitive material occurs. Accordingly, heretofore, the so-called masking method has been conventionally used, said method comprising the steps of forming an unsharp positive mask having a density distribution corresponding to the contrast which is opposite to, but symmetrical to the original negative, superimposing said positive mask and original negative, and then printing the image on a high gamma photographic paper by use of said superimposed transparencies, thus reproducing the image. This method will be illustrated below in detail.

In this conventional masking method, if it is desirable to obtain the most effective photographic print, it is necessary to form various unsharp positive masking films the contrast and diffused width of which are different from one another, to carry out test printing and reproduction of the said films many times, and to select the most desirable reproduction, thus entailing extraordinary treatment time and effort. Furthermore, according to the said masking method, although the diffused width can be varied, the characteristic of diffusivity cannot be varied, so that a special effect cannot be attained.

Recently, an improved apparatus, wherein the masking is carried out by using a cathode ray tube as a printing light source, and scanning of the original negative is achieved by use of an electron beam spot, has been invented and proposed. However, in such a masking system, the scanning of a light spot on a fluorescent screen of a flying spot cathode ray tube is carried out by a light spot having a large area so as to produce an unsharp brilliant mask, or a lens system is placed between the original negative film and a flying spot cathode 3,177,764 Patented Apr. 13, 1965 ray tube so as to cause defocusing, and scanning of the original negative film is carried out by a large light spot, thereby producing an unsharp brilliant mask. Conseguently, in the above system, the production of a monitor mage to be viewed such as is produced in the case of this invention cannot be attained. Although production of an unsharp positive masking film has become unnecessary, since the result of masking is known upon the completion of the photographic print, test printing must be carried out until a favorable result is obtained. It is possible to vary the diffused width, but since the characteristic of diffusivity is determined by the characteristic of diffusivity of a cathode ray tube or lens system, sharpening of an unsharp photographic print cannot be attained.

An object of this invention is to provide a photographic printing apparatus for an electronic masking enlarger system, wherein the above-mentioned disadvantages do not occur.

According to this invention, an electron beam is focused on a fluorescent screen of a cathode ray tube; a light spot is uniformly scanned on the said fluorescent screen by deflecting the said electron beam by electric currents or voltages the wave forms of which are symmetrically triangular, and the frequencies of which are slightly different from each other; an original negative positioned under the said fluorescent screen is similarly scanned by use of the said fluorescent screen as a light source, thereby converting the spatial lights and shades of the original negative into time-responsive light signals; a part of the said light beam is deflected so as to be projected to a photosensitive tube; the electric signal obtained from the said photosensitive tube is impressed on a low pass filter the characteristic of which is adapted to be variable; the output of the said filter is amplified; and the intensity modulation of a cathode ray tube is carried out by the negative polarity of the said amplified output, whereby a closed negative feedback circuit is composed. On one hand, the electric signal from the said photosensitive tube is fed into a monitor cathode ray tube which is synchronized, through a gamma collecting circuit, with the flying spot of the said cathode ray tube, thus causing intensity modulation and focusing of the positive observed image; and on the other hand, the other part of the light beam passed through the original negative is made to be focused on the photosensitive paper to form an image, thus carrying out exposure.

According to the photographic enlarger of this invention, since a positive image corresponding to the print to be produced is focused, without treatment such as printing and development of the photographic paper, on the fluorescent surface of a monitor cathode ray tube, printing conditions such as contrast and diffused width of making image and characteristics thereof can be controlled and viewed while the masking effect is being observed, whereby optimum sharpness of the print and other effects can be attained at one time without repetition of test printing, in accordance with said conditions or by operating the printing mechanism of electronic tube system in direct response to said conditions.

Differing from the conventional prior art, the apparatus of this invention relates to a system wherein the area of the scanning light spot is made to be sufficiently small, and the frequency characteristic of the intensity modulating feedback signal applied to the flying spot cathode ray tube is passed through means by which it can be adjusted by passing the said signal through a low pass filter having a characteristic as will be described hereinafter, to a signal the high frequency band or particular frequency band of which is cut off, whereby the unsharp masking image caused by the electric signal is produced on a mentioned advantageous characteristics are obtained. These advantageous characteristics are obtained for the following reason.

Since the area of the scanning light spot is sufficiently small, the electric signal produced by the radiated light beam, after passing through the original negative film, on a photosensitive tube has fidelity ranging over a sufiiciently high frequency band. Consequently, if the output signal of the said photosensitive tube is led out prior to its entering the filter, signals capable of resolving variation of the sufficiently detailed contrast of the negative can be obtained. Accordingly, when the said signals are impressed on a monitor cathode ray tube, the light spot scanning of which is made to be synchronized with the flying spot cathode ray tube to carry out intensity modulation, it is possible to produce an image which is resolved in sufiiciently detailedcontrast on the fluorescent screen of the monitor cathode ray tube. In this case, stages of the amplifying set are so selected that the signal to be applied to the monito cathode ray tube has a polarity adapted to represent a positive image on the fluorescent screen. This signal is affected by the said masking image as in the case of the photographic paper. Accordingly, if a gamma compensating circuit is inserted in the monitor amplifier, and the gamma of the monitor device including a monitor cathode ray tube is made to be coincident with the gamma of the photographic paper, an image which is the same as the printed image can be produced on the fluorescent screen of the said monitor cathode ray tube. That is, according 'to the apparatus of this invention, it is possible to control the masks contrast, fidelity (diffused width) and characteristic curve of the fidelity (diffusivity) while viewing the fluorescent screen of the monitor cathode ray tube and to obtain accurately the exposure level, and it is also possible to obtain an optimum result immediately, because the above-mentioned control and exposure level are automatically responsive to the desired printing conditions. Thus, time and effort which have been conventionally wasted become unnecessary, and, moreover, a desirably reproduced print to which optimum masking is applied can be obtained by a very simple treatment in a shorter time than with the conventional photographic enlarger.

Since the image of the monitor cathode ray tube is always stationary, when a fluorescent material capable of producing a residual light for a long time is used as the fluorescent material of the monitor cathode ray tube,

flicker caused by slow scanning speed can be prevented.

The above and other objects, advantages and features of this invention will become more apparent from the following description of an illustrative embodiment taken in connection with the drawings, in which:

FIG. 1 shows characteristic curves representing optical transfer functions, representing masking effect;

FIG. 2 shows characteristic curves representing sharpening of an unsharp photograph;

FIG. 3 shows characteristic curves representing optical transfer functions, representing compressing of the dark region of an objected contrast;

FIG. 4 is a block diagrammatic view showing the construction of the apparatus according to this invention;

FIG. 5 is a block diagrammatic view showing the construction of the apparatus of FIG. 4 to which a monitor device is added as a separate part;

' fluorescent screen of the cathode ray tube, and the above- This function is generally represented by a curve plotted with spatial frequency (density of lines per unit length of image surface) as the abscissa and fidelity as the ordinate.

For making the description understandable, curves of negative and printing light source in FIGS. 1, 2, and 3 are for images printed on photographic paper with ideal characteristics.

FIGjl shows a curve I for a photographed image having an ideal characteristic, wherein gross contrast as well as detail contrast are reproduced with fidelity, thereby producing the same image as the object.

In the conventional photographic method, since the above-stated unsharpness occurs, fidelity of detail contrast decreases, thus causing production of a photographed image having a characteristic such as is shown in FIG. 1 by curve II. When there is formed a light source having an intensity distribution (density) of symmetrical contrast which is opposite to that of the light passing through the original negative having characteristic curve II and having a fidelity as shown in FIG. 1 by the curve III, and the said original negative is located below the said formed light source so as to focus the photographed image on the photographic paper, the printed image has a characteristic such as shown in FIG. 1 by the curve IV.

That is, it becomes possible to obtain a photographed gross contrast of the original negative. By printing the said image on a high gamma photosensitive paper, a sharp image having an improved fidelity such as is shown by the characteristic curve V, which is much superior to the curve II, will be reproduced.

As described above, in the case of photographing under an adverse condition of illumination, since the ratio between the maximum and minimum of intensity of the light transmittedfrom original negative exceeds a linear band, it is preferable to make the gross contrast fall within the linear band by decreasing the said gross contrast. This improved state is shown in FIG. 3, wherein the characteristic curves I, II and III are optical transfer function curves representing, respectively, the characteristic of fidelity of the original negative, the light source, and the reproduced image. In a manner such as described above, a reproduced image which is identical to that in the case of illumination under controlled condition can be obtained by decreasing the gross contrast.

Characteristic curves for showing sharpening of diffused photograph are shown in FIG. 2, wherein the curve I shows the optical transfer function of an original negative. According to the curve I it will be understood that inter.- mediate detail parts substantially disappean When an original negative having a characteristic function as shown n FIG. 2 by curve I is located below a light source havmg a characteristic curve II in FIG. 2, and an imageis reproductively focused on a high gamma photosensitive paper, a remarkably sharp image can be reproduced shown by the characteristic curve III in FIG. 2.

' In order to obtain the above-mentioned effect in the apparatus of this invention, a flying spot cathode ray tube is used as the light source; a light spot is sharply focused on a fluorescent screen; scanning is carried out by the said light spot having suificie'ntly small area; and a low 'pass nlter'havmg frequency characteristics corresponding to an optical transfer function such as those shown in FIGS. 1, 2 and 3 is provided in the intensity modulation circuit of the cathode ray tube to carry out intensity modulation of the said cathode ray tube, whereby a light source having an optical transfer function curve such as those shown in FIGS. 1, 2 and 3 is produced. By varying the characteristic of the low pass filter, other particular characteristics besides the above-mentioned characteristics can be obtained. r

Referring to FIG. 4, a flying spot cathode ray tube '23 is provided with a control grid 24, a cathode 25, an accelerater terminal 10, a fluorescent screen 13, and a deflection coil 6. An accelerating voltage is applied, by way of an electric conductor 9, to the accelerater terminal 14) from a high voltage power supply circuit 4. This circuit 4 is, for example, of a type which rectifies a high alternating voltage produced by a high frequency oscillator. The deflection coil 6 consists of two pairs of coils which are perpendicular to each other, and said pairs are supplied with respective scanning deflection currents from a deflection oscillator 3 respectively through conductors 7 and 8. The frequencies of currents supplied to the defleotion coils from the deflection oscillator 3 are slightly different from each other, and both said currents have a symmetrical triangular wave form. Accordingly, the electron beam emitted from the cathode 25 carries out uniform scanning of a light spot on a fluorescent screen in the form of a parallel linear line in diagonal directions. This light spot is focused on the fluorescent screen by electromagnetic or electrostatic focusing. An original negative 12 held by a supporter I1 is located below the fluorescent screen 13 and is scanned by the light spot on the fluorescent screen in the same manner as described above. A part of the light beam passed through the original negative is reflected at a semi-transparent mirror '16, collected by an optical integrator 14, and then projected into a photosensitive tube through a difluser 15. This input to the said tube 5 is subjected to an intensity modulation in accordance with the density of the original negative as a result of the light from the scanning light spot having passed through the negative, whereby the spatial frequency on the original negative is converted to time frequency. The electric signal obtained by the photosensitive tube 5 is transmitted, by way of the electric conductor 22, to the low pass filter 2. In this filter, one of the predetermined characteristics is selected so as t impart suitable fidelity. The output of the low pass filter is transmitted, by way of an electric conductor 21, to the feedback amplifier 1 so as to be amplified thereby. This amplified output is applied, by way of an electric conductor 20, to the control grid 24 of the cathode ray tube. The electron beam emitted from the cathode 25 is controlled by a control signal applied to the control grid 24, whereby intensity modulation is achieved, This intensity modulation is carried out by the use of such a polarity as will make a high density part of the original negative bright. On the other hand, a device \28 is mechanically connected to the device 26, which will be explained later, and converts the electric signal, transmitted through conductor 27, to a function of the enlargement ratio, and can be, for example, a potentiometer. Device 26 is a cam mechanism to convert the up-and-down movement of device 28, which is to change the printing ratio, to revolving movement by an appropriate means, so that the movement of potentiometer, i.e, device 28, is converted in proportion to a reciprocal number of the enlargement ratio. The output of device 28 is introduced into an integrator 3% through an electric conductor 29. This integrator 30 integrates the electric signal until the exposure value reaches a predetermined value. A relay is operated by the said integrator when the integrated value of the said integrator becomes a predetermined electr-ic voltage corresponding to the said predetermined eX- posure value. This relay extinguishes, when it is operated, the light spot of the cathode ray tube, thus terminating exposure, In the above means, the best exposure can be given to a photosensitive paper by setting said predetermined electric voltage of the integrator in ac cordance with a proper exposure value of the photosensitive paper. In FIG. 8 is shown an integrating circuit diagram of a representative integrator, wherein electronic vacuum tubes V V V and V resistors R R R and a condenser C are provided. On the other hand, a light beam passed through a semitransparent mirror 16 is focused, by a lens 17, on a piece of photosensitive paper 18 supported on the support 19 so as to form an image on the said paper, whereby a desirable result can be obtained by exposure.

In FIG. 5 is shown an apparatus to which a monitor device is added as a separate part, wherein the apparatus corresponding to that of FIG. 4 is shown to the right of the broken line. The output signal of the photosensitive tube of the apparatus of FIG. 4 is supplied, by way of an electric conductor 34, to a gamma compensating circuit 36. The output of the said circuit 36 is fed, by way of an electric conductor 35, to an amplifier 37 and then fed, by way of an electric conductor 38, to the control grid 39 of a monitor cathode ray tube 41 where intensity modulation is carried out. In such a manner as is described above, a positive monitor image is produced on the fluorescent screen 45 of the monitor cathode ray tube 41 in a manner such as will be described in connection with the illustration of FIG. 6. An accelerating electric voltage is supplied to an accelerator terminal 43, through an electric conductor 44, from a high voltage power supply circuit 50 having the same function as that of the high voltage supply circuit 4 in FIG. 4. A sweep signal is supplied, through electric conductors 32 and 33, to a sweep output amplifier 47 from a sweep generator 3 of the apparatus. The output of the said sweep output amplifier is supplied as a sweep signal to the deflection coil 42 through electric conductors 48 and 4& A protecting device 4-6 for protecting the fluorescent screen from damage acts to extinguish the light spot when the sweep signal supplied from the amplifier 47 stops, thus protecting the fluorescent screen from burning. In FIG. 6 is shown an apparatus provided with a monitor device according to this invention, wherein the flying spot cathode ray tube '23 is provided with a control grid 24, a cathode 25 and an accelerator terminal 10, a fluorescent screen 13 and a deflection coil 6. An accelerating voltage is supplied, by way of an electric conductor 9, to the accelerator terminal 10 from a high voltage power supply circuit 4 of a type which rectifies a high alternating voltage produced by a high frequency oscillator. The deflection coil 6 consists of two pairs of coils which are perpendicular to each other and which are supplied with scanning currents from a sweep generator 3, through electric conductors 7 and 8, respectively. The currents supplied to the deflection coils are of slightly diiferent frequencies and have symmetrical triangular wave forms. An electron beam emitted from the cathode 25 is modulated at a control grid 24- by a signal such as will be described hereinafter and then deflected by the deflection coil 6, whereby uniform scanning of the light spot on the fluorescent screen is carried out as a parallel linear line of diagonal direction. Since this scanning is random, the light projected on the photosensitive paper 18 is uniformly scanned without being accompanied by any striped pattern. The electron beam is focused on the fluorescent screen by electromagnetic or electrostatic focusing systems. For the deflection of the electron beam, electric voltages having symmetrical triangular wave forms and frequencies which are slightly different from each other may be used. The original negative 12 held by the supporter ll is located below the fluorescent screen, and the same scanning as mentioned above is carried out by using the said screen as a light source. A part of the light beam passed through the original negative is reflected by a semitransparent mirror 16, collected by an optical integrator l4, and then projected, through an optical diffuser 15, onto a photosensitive tube 5. This projected light is modulated in intensity, it being the light from the light spot which has passed through the parts of diflerent density of the original negative, so that spatial frequency on the original negative is converted to time frequency. The electric signal obtained by the photosensitive tube 5 is supplied, by way of an electric conductor 22, to a loW pass filter 2. This filter 2 includes various unit filters having different characters, and among the said unit filters r 7 any one having a character capable of imparting the most favorable fidelity to the reproduction is selected.

The output signal of the filter 2 is supplied, through the 7 electric conductor 21, to a feedback amplifier l and then to the control grid 24 of the cathode ray tube 23 through the electric conductor Ed. The contrast of the masking image of the cathode ray tube 23 can be varied by varying the amplification factor of the feedback amplifier 1. The above-mentioned selection may be carried out while viewing the monitor image. The control grid 24 controls the electron beam emitted from the cathode 25 in response to the output signal of the said amplifier 1, thus causing intensity modulation. This modulation is carried out in such a phase that when the quantity of light passed through original negative is large, the light spot is darkened, and when the said quantity is small the light spot is brightened. The electric signal transmitted by way of an electric conductor 27 is introduced into an exposure control device 28 which is responsive to exposure value and connected, by way of conductor 26, to an automatic focusing mechanism for the enlarging lens. The signal introduced into the device 28 is converted to a function of the enlarging factor and then supplied to the integrator 30 through an electric conductor 29. This integrator integrates its input signal until the exposure value reaches a predetermined value and operates a relay when the integrated value reaches a voltage corresponding to a suitable exposure value. The said relay extinguishes, upon its operation, the light spot of the cathode ray tube, thus stopping exposure. A part of the output of the photosensitive tube is applied, by way of an electric conductor 34, to a gamma compensating circuit 315 in which the gamma of the photosensitive paper and the gamma of the monitor circuit including the cathode ray tube are corrected so as to be the same. The output of the compensating circuit 36 is introduced, through an electric conductor 35, into an output amplifier 37 and then applied, through an electric conductor 38 to the control grid 39 of a monitor cathode ray tube 41.

The monitor cathode ray tube d1 consists of a cathode 4d, a control grid 39, a fluorescent screen 45, an accelerator terminal 45, and a deflection coil 42. This coil 42 consists of two pairs of coils which are perfrom the cathode 40 is modulated by the electric signal supplied to the control grid 39 from the output amplifier 37 and deflected by the deflection coil 42 in synchronism with that of electron beam of the flying spot cathode ray tube, whereby the same monitor image as the positive image to be produced on the photosensitive paper is produced on the fluorescent screen 45. Maximum value of the image contrast is indicated 'on a logarithmic indicator 51 by a peak rectifier 52. This indication is identical to the indication of the exposure linear band on the photosensitive paper, so that when 'contrast of the masking image is controlled so that the said indication becomes lower than the exposure linear band, irregularity of the contrast is prevented.

On the other hand, the part of the light passed through the original negative 12 is made to pass through a semitransparent mirror "16 and then is focused, by way of a lens17, on the light sensitive surface of a piece of photosensitive paper 13 held in its supporter 19, whereby printing is achieved, thus causing a desirable, favorable result.

FIG. 7 shows a perspective view of a practical embodiment of the apparatus of this invention. Referring to FIG. 7, all of the electric circuits and heater circuits-of the electronic tubes are closed by closing a power switch 68, and anode voltage is supplied to all of the electric circuits by closing a power switch 69. Exposurevalue is adjusted by a knob '70 to a predetermined position corresponding to the photosensitive paper to be used. A knob 64 for correcting the gamma of the monitor is adjusted to a position corresponding to the gamma of the photosensitive paper. After the above-mentioned adjustments, an original negative is inserted into a window 54 while being held by its supporter. Enlargement and reduction of the image is achieved by raising or lowering a head part consisting of a box 53 enclosing the flying 7 spot cathode ray tube 23 therein, a box 55 enclosing a semitransparent mirror and other parts therein, a bellows 56, a lens cylinder 5-7, and other parts, said movement of the said head part being along a support an and produced by an electric driving motor enclosed .under table '76. nob 7-2 is the starting switch for the said motor. By turning the said switch 7-2 forward and rearward, the head part is moved, respectively, upward and downward. A metal belt 59 extending over pulley 53 connects the head part with a balance weight in a support 60. Even when the head part is moved up and down, focusing of the original negative on the photosensitive paper is carried out automatically by the movement of a lens cylinder caused by an automatic focusing cam 6-1. When an exchange switch 73 for exchanging exposure from automatic operation to manual operation, or vice versa, is turned to the manual position after determination of the head position, and an exposure starting button '75 'is pushed, a monitor image is produced on the fluorescent screen of the monitor cathode ray tube 45. The image of the said monitor cathode ,ray tube is adjusted'by a knob 65. 7

Then, contrast and characteristic of fidelity of the masking image are controlled by knobs "74 and '71, respectively, while the meter 51 and monitor image are being viewed so as to obtain a good image. The knob '74 is adapted .to vary the contrast of the masking image, thus causing variation of the amplification factor of the feedback amplifier 1. The knob 71 is adapted to vary the characteristic of fidelity, .thus causing variation of irequency characteristic of the low. pass filter 2.

After production of a favorable monitor image by the above-mentioned controls, a push button 75 is pushed so as to extinguish the image, and then a piece of photosensitive paper is clamped on the supporting board 1?, and the knob '73 is turned to the automatic position. After establishment of the above adjustments, exposure is commenced by pushing the push button '75 and is automatically stopped. The head part and other electric circuitsare connected by cables 62 and 63. The table and box enclosing the electronic circuits are, respectively, designated by reference numerals '76 and 66.

What is claimed is: g V

1. A photographic printing apparatus, comprising a cathode ray tube, a means coupled to said cathode ray tube for focusing an electron beam on the fluorescent surface of said cathode ray tube to produce a light spot on said surface, a scanning means which produces at least two deflecting electric signals each having a symmetrical triangular form and having frequencies which are slightly different from each other, said scanning means being coupled to said cathode ray tube for making said light spot scan an original negative disposed in front of the fluorescent surface so that the density variations of the original negative are converted to time differentiated intensity variations, a means in the path of said light with the time differentiated intensity variations therein for projecting a part of said light from said path, a photosensitive tube into which said part of said light ,is projected and which produces a continuous electric signal, a low pass filter having means for varying the characteristics thereof and to which said photosensitive tube is con- 9 of the amplified output of said amplifying means, whereby a closed negative [feedback circuit is formed, and focusing means in the path of said light for focusing the remainder of said light passed through the original negative onto a piece of photographic paper.

2. A photographic printing apparatus, comprising a cathode ray tube, a means coupled to said cathode ray tube for focusing an electron beam on the fluorescent sunface of said cathode ray tube to produce a light spot on said surface, a scanning means which produces at least two deflecting electric signals each having a symmetrical triangular form and having frequencies which are slightly different from each other, said scanning means being coupled to said cathode ray tube for making said light spot scan an original negative disposed in front of the fluorescent surface so that the density variations of the original negative are converted to time differentiated intensity variations, a means in the path of said light with the time differentiated intensity variations therein for projecting a part of said light from said path, a photosensitive tube into which said part of said light is projected and which produces a continuous electric signal, a low pass filter having means for varying the characteristics thereof and to which said photosensitive tube is connected to supplythe output signal of said tube to said filter, an amplifying means connected to said filter for amplifying the output of said low pass filter, an intensity modulating means coupled to said cathode ray tube for imparting an intensity modulation to said cathode ray tube and to 10 which intensity modulation means said amplifier is connected to feed thereto the negative polarity of the amplified output of said amplifying means, whereby a closed negative feedback circuit is formed, a gamma correcting circuit connected to said photosensitive tube to receive the electric signal from the said photosensitive tube, a monitor cathode ray tube having means for scanning the electron beam thereof in synchronism with the said first mentioned cathode ray tube and connected to said gamma correcting circuit so as to receive the said electric signal through the said gamma correcting circuit, whereby the said monitor cathode ray tube is subjected to intensity modulation so as to produce a positive image and focusing means in the path of said light for focusing the remainder of said light passed through the original negative onto a piece of photographic paper.

References Cited by the Examiner UNITED STATES PATENTS 2,691,917 10/54 Curry.

2,842,025 7/5 8 Craig 88-24 2,947,810 8/ 60 Horsley.

2,960,019 1=1/-60 Craig 8824 X EVON C. BLUNK, Primary Examiner.

EMLL G. ANDERSON, NORTON A-NS'HER,

Examiners. 

1. A PHOTOGRAHPIC PRINTING APPARATUS, COMPRISING A CATHODE RAY TUBE, A MEANS COUPLED TO SAID CATHODE RAY TUBE FOR FOCUSING AN ELECTRON BEAM ON THE FLUORESCENT SURFACE OF SAID CATHODE RAY TUBE TO PRODUCE A LIGHT SPOT ON SAID SURFACE, A SCANNING MEANS WHICH PRODUCES AT LEAST TWO DEFLECTING ELECTRIC SIGNALS EACH HAVING A SYMMETRICAL TRIANGULAR FORM AND HAVING FREQUENCIES WHICH ARE SLIGHTLY DIFFERENT FROM EACH OTHER, SAID SCANNING MEANS BEING COUPLED TO SAID CATHODE RAY TUBE FOR MAKING SAID LIGHT SPOT SCAN AN ORIGINAL NEGATIVE DISPOSED IN FRONT OF THE FLUORESCENT SURFACE SO THAT THE DENSITY VARIATIONS OF THE ORIGINAL NEGATIVE ARE CONVERTED TO TIME DIFFERENTIATED INTENSITY VARIATIONS, A MEANS IN THE PATH OF SAID LIGHT WITH THE TIME DIFFERENTIAL INTENSITY VARIATIONS THEREIN FOR PROJECTING A PART OF SAID LIGHT FROM SAID PATH, A PHOTOSENSITIVE TUBE INTO WHICH SAID PART OF SAID LIGHT IS PROJECTED AND WHICH PRODUCES A CONTINUOUS ELECTRIC SIGNAL, A LOW PASS FILTER HAVING MEANS FOR VARYING THE CHARACTERISTICS THEREOF AND TO WHICH SAID PHOTOSENSITIVE TUBE IS CONNECTED TO SUPPLY THE OUTPUT SIGNAL OF SAID TUBE TO SAID FILTER, AN AMPLIFYING MEANS CONNECTED TO SAID FILTER FOR AMPLIFYING THE OUTPUT OF SAID LOW PASS FILTER, AN INTENSITY MODULATING MEANS COUPLED TO SAID CATHODE RAY TUBE FOR IMPARTING AN INTENSITY MODULATION TO SAID CATHODE RAY TUBE AND TO WHICH INTENSITY MODULATION MEANS SAID AMPLIFIER IS CONNECTED TO FEED THERETO THE NEGATIVE POLARITY OF THE AMPLIFIED OUTPUT OF SAID AMPLIFYING MEANS, WHEREBY A CLOSED NEGATIVE FEEDBACK CIRCUIT IS FORMED, AND FOCUSING MEANS IN THE PATH OF SAID LIGHT FOR FOCUSING THE REMAINDER OF SAID LIGHT PASSED THROUGH THE ORIGINAL NEGATIVE ONTO A PIECE OF PHOTOGRAPHIC PAPER. 